Yang Fluidization, Solids Handling, and Processing

Three-Phase Fluidization Systems 605

3.2Chemical Pulping of Wood Chips

Pulping of wood chips refers to the process by which the wood chips are degraded into fibers by removal of the lignin from the cellulose and hemicelluloses. The lignin is a highly polymerized substance that holds the cellulose fibers together. The two primary methods of pulping are chemical and mechanical. Chemical pulping consists of cooking the wood chips with the appropriate chemicals in an aqueous solution at elevated temperature and pressure. The chemicals used in chemical pulping can be either alkaline (kraft process) or acidic (sulfite process). The alkaline kraft process has become the chemical pulping method of choice because of advantages in chemical recovery and pulp strength. However, the formation of organic sulfides in the kraft process has caused environmental concern due to their escape as malodorous gases. A brief overview of the kraft process will be given here. Details on mechanical pulping and the sulfite process can be found in Smook (1992).

In the kraft process, an aqueous mixture of sodium hydroxide (NaOH) and sodium sulfide (Na2S) is used to break the lignin away from the cellulose fibers. The chemical reactions that occur are complex and not completely understood. The alkaline attack essentially breaks down the lignin molecules into smaller fragments whose sodium salts are soluble in the solution. The cooking of the wood chips is completed when the kappa number, a measure of the residual lignin content in the pulp, of the pulp has achieved a desirable level. The primary objective in the cooking of wood chips is to consistently obtain the desired kappa number with a minimal amount of cellulose degradation and a minimal level of unusable (reject) material, producing a pulp of uniform quality while working with unavoidable variations in the wood feedstock. The two factors that drive the kraft pulping reactions are alkali concentration and temperature. The rate of removal of the lignin (delignification) more than doubles over the range of normal cooking temperatures (155–175°C) with every 10°C increase in temperature (Smook, 1992). One drawback to the kraft process (and chemical pulping in general) is that while the method is able to remove a large portion of the lignin, a certain amount of degradation of the cellulose will also occur. Typical yields from kraft processes are between 40% and 50% of the original wood substance compared with 85% to 95% from mechanical pulping. Various factors that affect the chemical cooking of wood chips via the kraft process are listed in Table 7.

606 Fluidization, Solids Handling, and Processing

Table 7. Factors Affecting Kraft Cooking (After Smook, 1992)

·Type of wood (hardwood or softwood)

·Wood chip quality (size and thickness uniformity)

·Wood chip moisture content

·Sulfidity of cooking liquor

·Strength of chemical application

·Liquor-to-wood ratio

·Temperature cycle

·Cooking time/Reaction rate

·Extent of delignification (kappa number)

·Residual alkali

The kraft process can be carried out in a reacting scheme that is batch or continuous. Advantages of batch and continuous schemes are delineated in Table 8. The vessel in which the cooking takes place is commonly referred to as a digester. The three-phase behavior in the digester is characterized by the wood chips immersed in the aqueous solution, with the gas phase being steam injected for heating purposes and/ or volatile organic gases produced as the reaction proceeds. Cooking in a batch digester generally takes 2–4 hours at a maximum temperature around 170°C, see Table 9. The maximum temperature is typically reached in 1.0 to 1.5 hours, allowing the cooking chemicals to penetrate the wood chips. The cooking reactions then proceed for up to 2 hours at the maximum temperature. The heating of the cooking mixture is conducted through direct steam injection or by forced circulation of the cooking chemicals through an external heat exchanger. Batch digesters are fabricated from carbon steel with a size range of 200 to 265 m3 and are capable of producing up to 19 tons of pulp per batch (Smook, 1992).

Three-Phase Fluidization Systems 607

Table 8. Advantages of Batch and Continuous Digester Schemes (After Smook, 1992)

Table 9. Cooking Conditions for the Kraft Chemical Pulping Process

The increasing demands on the pulp and paper industry to reduce emissions from bleaching operations has lead to modifications of the cooking process to provide a pulp that requires a decreased amount of bleaching. This is accomplished by decreasing the amount of residual

608 Fluidization, Solids Handling, and Processing

lignin content in the unbleached pulp by extended delignification in the cooking process. The most widely researched method to achieve extended delignification is the addition of additives, such as surfactants, anthraquinone, or polysulfide, to the cooking process. The additives work by either accelerating the delignification or enhancing the penetration and diffusion of the cooking chemicals into the wood chips (Chen, 1994). An example of a mechanism for improved penetration and diffusion through the addition of a surfactant was provided by Chen (1994). The surfactant, Chen (1994) stated, assisted in the removal of hydrophobic extractables that block the cooking chemicals from penetrating the chips. The uniformity of the pulp was also increased under these conditions. A desirable feature of extended delignification is the minimal capital cost requirement for digester modification or retrofitting (McDonough, 1995).

A requirement that has received attention in the literature is a provision for improved removal of the pulp from the digester at the completion of cooking. Traditionally, the pulp has been removed from the digester by blowing the contents under full digester pressure into an adjacent blow tank. This type of removal subjects the pulp to complex twoor three-phase flows with high velocities; the pulp experiences severe temperature and pressure drops and high velocity impacts with the blow tank walls. Therefore, pulp sampled from the digester at the end of cooking but before blowing demonstrated greater strength than pulp tested after the blowing process. Cyr et al. (1989) modified the discharge arrangement on a batch digester by adding dilution liquor and pumping out the contents of a cooled and depressurized digester at a low and controlled flow velocity. With this discharge arrangement, the strength levels of the discharged pulp approached those of the unblown material in the digester at the end of cooking.

Another area where a major research effort has been undertaken is in the automated control of batch digesters. Improved controlling devices for batch digesters are implemented to improve the uniformity of the pulp, reduce production costs, and save energy (Dumont, 1986). The control of batch digesters is complicated because of the inability to measure the kappa number of the pulp in the digester on-line (Lee and Datta, 1994). During a batch cook, the extent of delignification must, therefore, be conducted by secondary measurements. Recall that the kappa number is a measure of the residual lignin content in the pulp and a batch cook

Three-Phase Fluidization Systems 609

generally targets a set kappa number to reach at the end of the cook. The targeted kappa number depends on the time, temperature, and type of wood chips (hardwood or softwood) in the digester among other factors. The attainment of the targeted kappa number in the digester is critical in decreasing the chemical load in the bleaching of the pulp and, hence, reducing the environmental impact.

The secondary measurements on the extent of delignification focus on determining liquid properties (Paulonis and Krishnagopalan, 1988; Lee and Datta, 1994), such as liquid temperature, lignin concentration, solid concentration, and effective alkali, or also by estimating the extent of carbohydrate degradation during cooking (Alen et al., 1991). Lee and Datta (1994) provided a review of the various empirical models, based on secondary measurements, used to assist in the control of a pulp digester. The empirical nature of the models leads to difficulties in handling major changes in the process. Examples of control techniques include a nonlinear approach using liquid analysis (Lee and Datta, 1994), the use of neural networks (Dayal et al., 1994; Yeager, 1995) and continuous process improvement techniques (Saraiva and Stephanopoulos, 1992). Other procedures to improve and optimize pulp digester operation lie in the improvement of scheduling a set of batch digesters (Hvala et al., 1993; Rihar, 1994) and improvements in the heat economy of the digesters from batch to batch (Tikka et al., 1988; Petrov…i… et al., 1995). While a review on all the various types of control techniques is beyond the scope of this section, the highlighted control procedures/techniques are the most commonly encountered in the literature.

3.3Pulp Bleaching and Flotation De-inking

The objective of bleaching the pulp is to continue the delignification process started in cooking, thereby removing materials which contribute to the pulp color and leaving behind cellulose and hemicellulose which are inherently white. The bleaching process consists of a sequence of process stages, applying different chemicals in each stage. The most common chemicals used in bleaching stages are listed in Table 10. Based on the bleaching chemicals (i.e., chlorine, oxygen, ozone), the bleaching of pulp occurs in a three-phase mixture. The use of elemental chlorine (Cl2) has decreased in recent years because of environmental concerns about the

610 Fluidization, Solids Handling, and Processing

linkage of chlorine bleaching and dioxin in the effluent from the bleaching plant. Elemental chlorine-free (ECF) bleaching sequences, which substitute chlorine dioxide (ClO2) for elemental chlorine, are the primary choice presently in the paper and pulp industry. Environmental pressures to completely eliminate chlorine based products from bleaching sequences has lead the industry to explore new chemicals and processes to achieve totally chlorine-free bleaching (TCF) in the near future. The high oxidizing potential of ozone has put ozone bleaching technology at the forefront of TCF bleaching. The barriers to overcome in the application of ozone to TCF bleaching are the poor selectivity of ozone (ozone oxidizes cellulose as well as lignin), nonuniform bleaching with ozone that can adversely affect the pulp, and the high cost associated with the production of ozone (Nutt et al., 1993).

Table 10. Common Chemicals used in Bleaching of Paper Pulps

The bleaching reaction is traditionally conducted in vertical towers approximately ten meters in height with the pulp slurry mixed with the bleaching gas at the inlet and pumped continuously through the tower. The three-phase reacting mixture travels through the column, via upflow or downflow, as a moving bed or plug flow type system. The mixing of the gas and pulp slurry at the inlet, therefore, becomes critical in providing a

Three-Phase Fluidization Systems 611

uniformly bleached pulp. Coalescence of bubbles can lead to a decreased reaction rate because of a decrease in the surface area of the bubbles in contact with the pulp slurry and an increase in channeling of the gas phase through the reactor, which decreases the residence time of the gas phase in the reactor.

These bleaching reactions can be carried out under a wide range of weight percent of pulp or consistency (based on the oven dry weight of the pulp in the slurry) of the pulp phase. The choice of the percentage of pulp or pulp consistency at which the bleaching will take place is important due to several factors. The first is that the bleaching reaction occurs quickly once the bleaching chemical comes into contact with the pulp phase, especially in the case of ozone. This means that the bleaching reaction is mass transfer limited. The greatest resistance to mass transfer in a well mixed system occurs in the liquid phase that separates the gas bubble from the pulp fibers; i.e., in the mass transfer from the bulk gas phase to the surface of the fiber, see Fig. 4. Therefore, a reduction in the amount of liquid present (an increase in pulp consistency) decreases the resistance to mass transfer (White et al., 1993). Increasing the pulp consistency, however, renders the pulp less amenable to mixing in traditional mixers and more complicated designs for high consistency (25% to 28%) bleaching, in some cases, are required. An example of this, shown in Fig. 5, is the high consistency ozone bleaching paddle conveyor reactor developed by Union Camp Corporation, which is a horizontal, tubular reactor with shaft-mounted paddle-type internals (White et al., 1993). High and medium (10–14%) consistency bleaching reduces the volume of pulp slurry to be pumped and the amount of effluent to be treated at the cost of increased mixing requirements and, possibly, more complicated reactor design. Hurst (1993) demonstrated that the effectiveness of ozone as a bleaching chemical increased with increasing pulp consistency under similar reaction conditions (mixing intensity, reaction time). The downside of high consistency ozone bleaching is a loss in the viscosity of the pulp (cellulose); however, Kappel et al. (1994) reported that the strength properties were not significantly affected. Hurst (1993) also stated that increasing the reaction time for low consistency (3–9%) pulps provided results similar to the high consistency case.

Three-Phase Fluidization Systems 613

Flotation de-inking is becoming an important process in recycling of paper fibers as the demand of recycled fiber grows based on environmental and social concerns. In flotation de-inking, gas bubbles and flotation chemicals are introduced with a slurry of recycled paper into a flotation cell where the objective is for the ink particles to detach from the fibers and adhere to the gas bubbles, then, for the ink-laden bubbles to rise to the upper portion of the cell to be removed as a foam layer. The recycling of fibers by using flotation cells is typically conducted in a series of 6 to 10 cells for efficient ink removal, which is usually in the 50–70% range. Flotation cells for de-inking evolved from mineral flotation with the notable difference that flotation de-inking occurs at much higher Reynolds numbers than mineral flotation (Lindsay et al., 1995). Several designs of flotation cells are available (see Smook, 1992), with the primary performance improvement through the years of development being a decrease in power consumption (McKinney, 1995).

The hydrodynamic behavior of the flotation cell is of paramount importance in achieving efficient operation. The basis for flotation deinking is the hydrophobic nature of the ink particles and their ability, with the assistance of added chemicals, to adhere to the bubbles. Therefore, high bubble surface areas and the opportunity for bubble-ink particle collisions are required for efficient removal of the ink particles. Typical ink particles removed in flotation de-inking are in the size range of 20 to 200 μm. High volumes of air injected into the cell provides the required surface area for increased probabilities of bubble-ink particle collisions; however, excess turbulence can be detrimental to the overall performance because of remixing or detachment of the ink particles from the bubble (Seifert, 1994). Optimal circulation of the three-phase mixture is also required so that the bubbles carrying the ink particles reach the upper surface and are removed, not re-entrained into the cell. Pulp consistencies in flotation cells, with a range usually around 0.8–1.2%, are lower than those encountered in bleaching operations. The pulp slurry residence time in the cell is 5–20 minutes. Other factors affecting the efficiency of the flotation cell are the temperature and pH, which need to be optimized based on the chemicals and even the type of material to be recycled.

The chemicals added to the flotation cell assist in separating the ink from the fiber and enhance the ability of the ink particles to adhere to the bubble surface. A common chemical added to the cell is sodium hydroxide (NaOH) which causes the fiber to swell, thus releasing the ink particle

614 Fluidization, Solids Handling, and Processing

(Turvey, 1995). The addition of NaOH has the advantage of increasing the strength of the fibers, but the optimal pH of the cell may be significantly influenced by the addition. Other additives to the flotation cell are surfactants. The surfactants act by reducing the surface tension of the liquid, thus decreasing the bubble size to provide the high surface area and optimal bubble size for ink particle capture. The addition of any chemicals leads to consideration of their effects downstream from the flotation cell. The selection of chemicals, therefore, has to be performed carefully so that minimal adverse effects, both in the use of the recycled fibers and in the environmental impact of the effluent, are considered. Another consideration in the recycling of fibers is the effect of recycling on the quality of the pulp fibers. A review of this complicated topic is beyond the scope of this chapter, however, a recent review by Howard (1995) is recommended for the interested reader.

4.0CHEMICAL PROCESSING

4.1Introduction

This section covers recent advances in the application of three-phase fluidization systems in the petroleum and chemical process industries. These areas encompass many of the important commercial applications of threephase fluidized beds. The technology for such applications as petroleum resid processing and Fischer-Tropsch synthesis have been successfully demonstrated in plants throughout the world. Overviews and operational considerations for recent improvements in the hydrotreating of petroleum resids, applications in the hydrotreating of light gas-oil, and improvements and new applications in hydrocarbon synthesis will be discussed.

4.2Hydrotreating/Hydrocracking Petroleum Intermediates

The hydrotreating and conversion of petroleum resids into viable diesel and other lighter fuels has become important due to the large reserves of heavy crudes. Petroleum resids are the residual portion of the initial atmospheric crude oil distillation containing the higher molecular weight organics and many organic-metallic complexes. With stricter air pollution control and increasing mandates for the burning of cleaner fuels, hydrotreating becomes essential in the removal of the heavy metals,